1. Field of the Invention
This invention relates to a method and apparatus for continuously forming functional deposited films with a large area by decomposing and exciting starting gases through plasma reactions caused by a novel microwave energy applicator capable of generating a uniform microwave plasma over a large area. More particularly, the invention relates to a method and apparatus for continuously forming functional deposited films with good uniformity over a large area by drastically increasing the utilization efficiency of the starting gases and at high speed. By this, mass production of large area thin film semiconductor devices such as photovoltaic devices can be realized at low costs.
2. Description of the Prior Art
Electric power demand is now on the drastic increase on a worldwide scale. Studies have been made on how the electric power production should be conducted to meet the demand as one of worldwide problems. At present, electric power production has been made through hydroelectric power generation, steam power generation and atomic power generation. Among these power generation systems, the hydroelectric generation, which utilizes rain fall, is difficult in a given amount of stationary electric power production. The steam power generation makes use of so-called fossil fuels such as petroleum, coal and the like and most of the power consumption has been met by this generation system. However, the fossil fuel is limited in amount, coupled with another substantial problem that carbon dioxide inevitably discharged during the power generation will invite the global greenhouse effect. Thus, the necessity of conversion into other power generation systems has been discussed.
Under these circumstances, great attention has been paid to the atomic power generation with a tendency to increase a ratio of the atomic power generation in the electric power production. Since, however, the atomic power generation involves the great possibility of presenting a serious life-or-death question on all creatures, i.e. the radioactive contamination problem, the security of safety is essential and studies on this have been made globally.
In the light of this background, "solar power generation" has attracted great attention in recent years and a great number of proposals have been made on the solar power generation. However, all the proposed solar power generation systems have never enabled the power production from the standpoint of supplying the electric power demand. The hitherto proposed solar power generation systems can be broadly classified into the following two groups.
System A: sunlight is converged to heat water to boiling thereby generating steam (photo-thermo conversion), followed by power generation in boiler (thermoelectric conversion).
System B: electric power is generated by the photoelectric conversion through solar cells.
The solar generation system A is one wherein electric power is generated after two energy conversion stages with a low conversion efficiency of the system. Although small plant tests started, this system has not been in use yet.
On the other hand, with regard to the solar power generation system B, a number of systems have been proposed, among which some have been actually used as a power source such as of wrist watches, desk calculators and the like. The power generation apparatus for the purpose of electric power have been tested on a small scale but are now not beyond the range of investigation stages.
This is because there are several problems which have to be solved in order to stationarily produce electric power on a large scale, of which the greatest problem is whether or not solar cells having a large area enough to entail large-scale power production can be manufactured on an industrial scale since the capability of power generation is proportional to the area of the solar cell. The solar cell includes semiconductor junctions, such as so-called p-n junctions, p-i-n junctions and the like, in semiconductor layers used as important members. These semiconductor junctions are formed by superposing semiconductor layers of different conduction types, implanting one conduction-type dopant into a semiconductor layer of the other conduction type such as by ion implantation, or diffusing the dopant into the semiconductor layer by thermal diffusion. A number of materials used to form the semiconductor layer have been proposed but directed mainly to polycrystalline silicon (hereinafter referred to simply as "x-Si"), amorphous silicon (hereinafter referred to simply as "a-Si") and compound semiconductive materials.
In case where any material mentioned above is employed, production problems such as of uniformity, reproducibility and the like, problems on the photo-electric conversion efficiency and problems on production costs arise, so that production processes of solar cells with a large area in large amounts and element structures of the solar cell, which meet a huge power demand, have not been accomplished yet.
Especially, a solar cell panel using single crystal Si is fabricated by subjecting an Si wafer cut away from a silicon ingot to ion implantation to form p-n junctions and arranging the implanted wafers as closely as possible to connect them with one another. The production process of the solar cell element is not suitable for treating a large number of substrates with a large area. To begin with, the single crystal Si ingot with a larger diameter is very expensive. In the step of cutting away of the Si wafer, a great amount of non-utilizable portions are produced as a swarf. These cause the production costs of the single crystal Di solar cell to be increased. Any effective measure for solving the above problems has not bee found up to now. With regard to solar cells using polycrystalline Si, the photoelectric conversion efficiency is slightly better than the solar cell using a-Si, but the technique of controlling the size of crystal particles which is one of factors of determining the characteristics has not been accomplished. Moreover, the device fabrication process is almost the same as that for the single crystal Si solar cell and is not thus suitable for the mass production.
x-Si, which is either in a single crystal form or in a polycrystal form, has inherent properties of crystals that it is liable to break. Accordingly, for solar power generation in outdoor environments, severe protective materials are necessary. The solar cell panel units covered with a protective material become so high in weight that limitations are placed on the setting place and environment.
On the other hand, in the fabrication of solar cells with a large area by the use of a-Si, starting gases containing a dopant element, e.g. phosphine (PH.sub.3), diborane (B.sub.2 H.sub.3) and the like, are mixed with silanes used as a main starting gas and subjected to decomposition by glow discharge to obtain a semiconductor layer with a desired conduction type. A plurality of the semiconductor layers are successively deposited on a desired substrate to readily obtain semiconductor junctions. Thus, it is known that the cell can be fabricated more inexpensively than in the case using x-Si.
As for the glow discharge decomposition, the RF (radio frequency) glow discharge decomposition process has been technically established and, in fact, are in wide use. However, this process enables one to form the semiconductor film of relatively high quality at low deposition rate but is difficult in forming high-quality semiconductor films sufficient to function as a solar cell at high speed over a large area. Needless to say, the fabrication of solar cells on an industrial scale for supplement of the power demand will be very difficult.
As a process for forming deposited films of high quality at high speed, attention has been drawn to a plasma process using microwaves. Since the microwave has a short frequency band, it is possible to increase a power density in a film-forming chamber over conventional processes using RF. This process is suitable for efficiently initiating and sustaining a plasma.
For instance, U.S. Pat. Nos. 4,517,223 and 4,504,518 describe processes for the deposition of thin films on substrates at a low pressure by causing glow discharge by microwave power. According to these patents, these formation processes of deposited films are carried out at low pressure with attendant advantages such as a reduced number of recombinations of radicals which would cause deposited film characteristics to be lowered when great power is charged, a reduced degree of formation of fine powder, such as of polysilane, in the plasma and an improved film-forming speed. However, the transmission efficiency of the microwave energy to the plasma is not satisfactory. In the latter patent, it is disclosed to provide two microwave applicator means serving as slow wave circuits, which are not parallel to each other, in a plane parallel to a substrate. More particularly, it is disclosed that the central axes of the microwave applicator means should preferably be provided so that they are crossed in the planes parallel to the substrate and on a straight line which is at right angles to the direction of movement of the substrate and that in order to prevent the interference between the two microwave applicator means, the microwave applicator means are provided by transversely shifting with respect to the direction of movement of the substrate by a length corresponding to half the major side of a waveguide.
As an antenna system which is relatively easy in handling among microwave applicator means, microwave plasma CVD apparatus, as shown in FIGS. 18 and 19, are described, for example, in Japanese Patent Publication No. 57-53858 and Japanese Laid-open Patent Application No. 61-283116. In these apparatus, the antenna is surrounded by a cylinder constituted of a microwave-transmitting material and a film-forming chamber is air-tightly sealed by means of the cylinder. The microwave is introduced from outside of the film-forming chamber. As a result, the life of the antenna is prolonged by preventing deposition of the film on the antenna, ensuring generation of a high density plasma over a wide pressure range. In the apparatus shown in FIG. 19, within a reaction container 1901 are provided a substrate 1903 mounted on a substrate holder 1902 and a microwave applicator means. Reference numeral 1904 is a coaxial line serving as the microwave applicator means. The microwave power is supplied from a space 1905 provided by cutting away part of an outer conductor of the coaxial line 1904 into the reaction container 1901 through a microwave-permeable cylinder 1906. However, with this apparatus, it will be apparently difficult to uniformly form an a-Si film on the substrate with a large area and, in fact, the uniform formation over a large area is not particularly described. On the other hand, in the apparatus shown in FIG. 18, a reaction container 1801 has a rod antenna 1802 serving as a microwave applicator means, a gas feed port 1803, a gas exhaust port 1805 connected to a vacuum pump 1804, and substrates 1807 mounted on a quartz cylinder 1806. The microwave power generated with a microwave oscillator is transmitted through a waveguide 1808 and is discharged into a space surrounded by the quartz cylinder 1806 through the rod antenna 1802 and the microwave-transmitting member 1809 wherein a plasma is developed for the treatment with the plasma. However, because of inherent properties of the rod antenna where the microwave power or energy is radiated into the space through transmission of the rod antenna, the microwave energy is attenuated along the length of the rod antenna, making it difficult that the plasma is made uniform along the lengthwise direction.
With a cavity resonator system known as another type of microwave applicator means, there have been proposed several means for sustaining the uniformity of plasma. These proposals are reported, for example, in Journal of Vacuum Science Technology B-4 (January-February, 1986) pp. 295-298 and B-4 (January-February, 1986), pp. 126-130. According to these reports, a cylindrically-shaped cavity resonator-type microwave reactor called a microwave plasma disc source (referred to simply as MPDS) has been proposed. More particularly, the plasma is in the form of a disc and is included in the cylindrically-shaped cavity resonator as part of the resonator with its diameter being a function of the microwave frequency. These reports give evidence that MPDS is provided with a resonator length variable mechanism adapted to tune with the microwave frequency. In the MPDS designed to operate at 2.45 GHz, the plasma confined diameter is at most approximately 10 cm and the plasma volume is approximately 118 cm.sup.3. This is far from a large surface area. It is also reported that in a system designed for operation at the lower frequency of 915 MHz, a lower frequency source would provide a plasma diameter of about 40 cm with a plasma volume of 2000 cm.sup.3.
In the reports, it is further stated that operation at a lower frequency, for example, of 400 MHz will scale up the discharge to a diameter in excess of 1 m. In order to achieve the above purpose, a specific type of microwave oscillator for great electric power has to be developed. Even if such a microwave oscillator will be successfully assembled, the use of the oscillator would be difficult since limitation is placed on the industrially utilizable frequency according to the radio law in order to avoid disturbances on communication lines.
As yet another type of microwave applicator means, systems using electron cyclotron resonance (ECR) have been proposed in Japanese Laid-open Patent Application Nos. 55-141729 and 57-133636. These systems include electromagnets coaxially provided about a cavity resonator which becomes a plasma chamber. A magnetic field of 875 gausses is formed by means of the electromagnets in the vicinity of a microwave introducing window to establish electron cyclotron resonance (ECR) conditions, thereby enhancing a coupling rate of the microwave into a plasma and causing a high density plasma to be generated. The high density plasma is transferred along the dispersed magnetic field formed by means of the electromagnets, thereby forming a desired deposited film on a desired substrate.
This system is similar to the afore-described microwave plasma disc source system (MPDS) with respect to the use of the cavity resonator. However, part of the interior of the cavity resonator is occupied by the plasma in the MPDS system, from which the ECR system differs in that the plasma is filled in the interior of the cavity resonator and that the electron cyclotron resonance phenomenon is utilized.
In the academic circles in this field, a number of formations of various types of semiconductor thin films by utilizing the high density plasma formed by the ECS system have been reported. This type of microwave ECR plasma CVD apparatus has been already on the market.
In the processes using these ECR systems, the plasma is controlled by the use of a dispersed magnetic film applied from outside of the cavity resonator, so that the distribution of the magnetic field which is made by the electromagnets on the substrate surfaces becomes non-uniform, making it very difficult to form a uniform and homogeneous deposited film on a large-area substrate.
Japanese Laid-open Patent Application No. 63-283018 discloses a process wherein part of the above difficulty is overcome and the distributions of the film thickness and the film quality are improved. In this process, a magnetic field generator means (e.g. second electromagnets) other than the electromagnets provided about the cavity resonator is provided about the substrate by which uniformity in the film thickness and quality distributions is attained. However, the second electromagnet is too large in size. Even when the apparatus is arranged using so large a magnet, only the uniformity in the film quality and thickness of approximately 15 cm .phi. is obtained.
As described hereinabove, in addition of the extablishment of applicator means for high frequencies such as RF or microwave, it seem to be necessary to establish a continuous film formation apparatus for producing a solar cell device by successively depositing a plurality of semiconductor films with desired conduction types to form semiconductor junctions.
As an example of such a continuous film formation apparatus, there has been proposed an apparatus which includes a plurality of separate film-forming chambers for forming a plurality of semiconductor films, the chambers being connected through partition valves, and a pair of parallel flat plate-shaped RF electrodes provided in the respective film-forming chambers, wherein in the respective film-forming chambers, the respective semiconductor films are deposited by an RF glow discharge decomposition technique under conditions isolated from other film-forming chamber. More particularly, a so-called three chamber separation-type continuous film formation apparatus has been proposed as a continuous film formation apparatus of a deposited semiconductor device having p-i-n junctions. The respective film formation chambers for forming a p-type semiconductor layer, an i-type semiconductor layer and an n-type semiconductor layer are separated from one another by means of the partition valves wherein the respective layers are formed by the RF glow discharge decomposition method. During the deposition of the respective layers, the cycle of film formation, exhaustion, transfer and film formation is repeated and thus, it takes a long time for the film formation. In addition, the width of the substrate is limited by the partition valves. Thus, this apparatus is, by no means, a continuous film formation apparatus which is able to produce solar cell devices in amounts sufficient to substitute for current power generation systems.
In contrast, U.S. Pat. No. 4,400,409 discloses a roll-to-roll continuous film formation apparatus which is substantially free from the limitations on the film formation time and the width of the substrate and has thus a practical merit. According to this apparatus, a flexible band-shaped substrate having a desired with and an appropriate length is passed along which a plurality of RF glow discharge regions are provided. In the respective RF glow discharge regions, a semiconductor film mainly composed of a-Si is formed. The band-shaped substrate is continuously transferred along the lengthwise direction substantially horizontally. As a result, devices having semiconductor junctions constituted of a number of semiconductor layers corresponding to the number of the RF glow discharge regions can be continuously formed. In this patent, it is stated that the diffusion and incorporation of dopant gases used at the time of the formation of the respective semiconductor layers in other RF glow discharge regions are prevented by provision of gas gates. More particularly, the gas gates are used to separate the RF glow discharge regions from one another by means of slits serving as a gas separation passage. At the respective separation passages, a means for forming a stream of a scavenging gas such as, for example, Ar, H.sub.2 or the like is provided.
However, since the formation of the respective semiconductor layers is carried out by the plasma CVD method using RF (radio frequency). limitation is inevitably placed on an improvement of the film deposition rate while keeping the characteristics of the continuously formed film. For instance, even when the semiconductor film having a film thickness of, at most, 5000 angstroms, the film deposition rate is so slow that it is necessary to invariably initiate a given plasma along the travel direction of the elongated band-shaped substrate over a large area and to uniformly sustain the plasma. In doing so, an appreciable skill is required and it is difficult to generalize various related plasma control parameters. Moreover, the decomposition and utilization efficiencies of the starting gases used for the film formation are not so high, which is one of factors raising the production costs.
Japanese Laid-open Patent Application No. 61-288074 discloses a deposited film formation apparatus using an improved roll-to-roll continuous film formation process. In this apparatus, a flexible continuous band-shaped member set in a reaction container is formed at part thereof with a curved portion, to which an active species produced from starting gases in an activation space different from the reaction container is transferred. Thereafter, the active species is introduced into the reaction container to cause chemical interaction by thermal energy, thereby forming a deposited film on the inner surface of the curved portion of the band-shaped member. The deposition on the inner surface of the curved portion will make a compact apparatus. In addition, the use of the previously activated species will facilitate the film formation rate upon comparison with the case of the known deposited film formation apparatus.
This apparatus makes use of the deposited film formation reaction by the chemical interaction caused by application of the thermal energy. Accordingly, the distance between the reaction container of the deposited film formation apparatus and the activation space and the film forming conditions are limited depending on the manner of supplying the thermal energy, the likelihood of the reaction between the active species and other molecules and the life of the active species before deactivation. This results in a difficulty in forming a large area.
The thin film semiconductors have been appropriately applied not only to the afore-described solar cells, but also to thin film semiconductor devices requiring a large area or an elongated film such as thin film transistors (TFT) for driving picture elements of liquid crystal displays, photoelectric conversion elements for contacted image sensors and switching elements. In practice, some thin film semiconductors have been employed as a key component for the image input and output apparatus. However, provision of a novel deposited film formation process capable of forming films of high quality and good uniformity with a large area at high speed will widespread the thin film semiconductors for general purposes.